Effects of Expanded Polytetrafluoroethylene as a Packaging Material on the Appearance and Texture of Microwave-Baked Soy Cookies

Abstract

The feasibility of expanded polytetrafluoroethylene (ePTFE) as a packaging material for microwave-baked soy cookies was investigated in comparison with polytetrafluoroethylene (PTFE) and a susceptor incorporating elemental-metal into a receptacle (EMIR). Physical properties such as the dimensions, color, hardness, and the specific gravity of the products were measured. Soy cookies enclosed in ePTFE showed golden brown color evenly distributed with textural measurements compatible to regular chocolate chip cookies. Soy cookies microwaved in PTFE and EMIR yielded inferior physical attributes. ePTFE was capable of adequately retaining the moisture of soy cookies, indicating the potential as a packaging material for microwavable dough-based products.

Keywords:

• Soy flour
• Color
• Dimensions
• Hardness
• Susceptor

INTRODUCTION

Microwave energy, with its unique high-frequency that penetrates and generates heat within the product, has been shown capable of rapidly reducing the moisture content[1–3] and has offered many advantages over conventional processing methods for food applications such as cooking, thawing, blanching, dehydration, pasteurization, sterilization, and tempering.[4] A number of studies have been conducted to determine the effects of microwave energy on baked goods, including yeast raised breads,[5–7] cakes,[8–11] puff pastries,[12] and biscuits made from canned dough,[13] or from freshly prepared wheat flour.[14] However, while the market for microwave foods continues to grow, the use of microwave energy in baking is limited and remains a challenge for the industry, primarily due to the lack of crust formation and surface browning.[15,16]

The inability of microwave ovens to induce browning has been attributed to the inherent low product surface temperature resulted from the cool ambient temperature inside a microwave oven, which consequently prevents flavor and color compounds to be produced by Maillard browning reactions and caramelization of sugars.[17–19] Unlike conventional baking, in which heat is transferred mainly by convection from the heating media and by radiation from oven walls to the product surface, followed by conduction to the center,[20] in microwave ovens microwaves heat is generated by the interaction of microwaves with charged particles and polar molecules. This accelerated heating process provides for a higher quality product in terms of nutritional content, but, on the other hand, inevitably reduced the time required for critical baking steps, including starch gelatinization, starch conversion by enzymes (such as α-amylase) or expansion of dough, and final setting of the dough into a rigid crumb structure.[16]

Many attempts have been made to improve microwave baking, ranging from the development of baking utensil[21] to the formation of browning compositions.[22,23] After Seiferth[24] invented a disposable food receptacle for use in microwave cooking, extensive efforts have been devoted to the incorporation of microwave susceptors, which include a substrate and a coating of electrically conductive particles applied to the substrate, into food packaging.[25–28] In addition to the paperboard layer, the susceptors usually consist of a layer of adhesive, a polyester film, and an aluminum layer to shield areas of the food that tends to be overcooked.[29] Most recent developments seem to agree that food packages consist of metallized plastic film laminated to paperboard on top of which, or within which, the product is placed could improve crust formation and browning during microwave baking.[30–32]

Nevertheless, as the susceptors absorb microwave energy, convert it to heat, and transfer it to the product by conduction or radiation, localized areas of high temperature on the food surface are created. Even cooking and crisping of food are achieved by transferring the energy throughout the product. When the temperature of the susceptors reach 200–260°C, water is evaporated, and the product becomes crisp and brown.[33] However, besides crust formation and browning, the quality of microwave baked products are also greatly hindered by the reduced height of the product, dense or gummy texture, crumb hardness, and an undesirable moisture gradient along a vertical axis in the final baked product.[9,34–37] Furthermore, since moisture movement during microwave heating of solid moist food materials is considered to result from pressure and concentration gradients, evaporation at the surface is more critical in microwave heating than in conventional heating because more moisture moves from the interior.[38] Therefore, it is postulated that, in addition to desirable electrical properties such as surface resistivity, the ideal susceptors should also provide adequate moisture permeation during microwave baking.

The idea of producing microwaveable cookie dough is not new, nor the use of susceptors for heating of garnished flat dough in microwave oven, and, in fact, has resulted in several U.S. patents.[39–41] However, considerable discrepancies still exist while using soy flour as the base for the formulation of microwave baked cookies. As the first step towards the development of a gluten-free, nutritious product with desirable appearance and texture properties, in this study, a soy-based cookie product was created and subjected to three types of packaging films differing in electronic conductivity and moisture permeability to characterize the effects of these packaging materials on the appearance and texture of the enclosed soy cookies during microwave baking.

MATERIALS AND METHODS

Raw Materials

Dry, lecithinated soy flour (Soyarich® 115W, Central Soya Company, Inc., Fort Wayne, IL, USA) made from 200 mesh defatted soy flour with 15% added lecithin was used as the base material for the soy cookies. For the formulation of a low-fat product, Hershey's® low-fat, semi-sweet chocolate chips (Hershey Foods Corporation, Hershey, PA, USA) and a low-fat spread (I Can't Believe It's Not Butter® Spread, Unilever Bestfoods North America, Englewood Cliffs, NJ, USA) were employed. Other ingredients required to produce the soy cookies, including granulated sugar, brown sugar, shortening, eggs, baking soda, and salt, were purchased from a local supermarket. For comparison, commercial wheat flour was used in the formulation of the wheat flour group.

Packaging Films

Three types of packaging films qualified for use in contact with food in compliance with FDA Regulation 21 CFR 177.1550 were employed. Made from materials with different electronic conductivity and moisture permeability, the packaging films tested in this study included: (a) expanded polytetrafluoroethylene (ePTFE) sheets, which has been extensively used in a variety of applications due to its elasticity and increased porosity and high dielectric properties compared to PTFE,[42] in addition to its inherent hydrophobic surface[43]; (b) regular PTFE sheets, better known by the trade name Teflon® that has been used to make non-stick cooking pans due to its superior chemical stability, high thermal stability, high hydrophobicity, and low surface tension[44]; and (c) elemental-metal incorporated into a receptacle (EMIR), a patented thin layer susceptor consisting of a metalized/aluminized plastic film laminated to a rigid paperboard with the use of an adhesive material.[24,45,46] Each of the packaging films was carefully trimmed into rectangular pieces (12 cm × 18 cm) prior to use.

Dough Preparation

The ingredients used for preparation of the cookies were: soy flour (33.5%), semi-sweet chocolate (22.3%), granulated sugar (11.2%), brown sugar (11.2%), shortening (7.4%), low-fat spread (7.4%), eggs (6.6%), baking powder (0.3%), salt (0.1%), and water. Sugar and spread were creamed for 10 min in a desktop mixer (KitchenAid model K45, Hobart Manufacturing Co., Troy, OH, USA). Then water was added with mixing for an additional 5 min. Salt and baking powder were added in the same manner to the soy flour and mixed for 10 min. After addition of all ingredients, mixing continued for another 5 min. The dough was placed in polyethylene bags to rest for ca. 1 h, flattened manually using a wooden roller into a sheet about 3.00 ± 0.05 mm thick, and then cut into spherical pieces measuring 4.5 cm in diameter (ca. 19.0 g). To compare the texture and appearance of the soy cookies with chocolate chip cookies available on the current market, wheat flour instead of soy flour was prepared following the same procedures.

Baking in Microwave and Conventional Oven

Spherical cookie dough was placed at the center of the rectangular packaging film, which was then rolled up to form a tube along the short edge (12 cm), allowing ca. 2 cm overlap on the long edge so that the cookie dough was centered in the enclosure. With the overlapping seam of the packaging film kept underneath the enclosed cookie dough, microwave baking was conducted in a domestic Sharp® microwave oven (model R3A96, Sharp Electronics Corp., Mahwah, NJ, USA) at 850 W and 2450 MHz at 50% power for 75 sec. For comparison, the wheat flour dough used to manufacture commercially available chocolate chip cookies was placed on a stainless steel cookie sheet and baked in a free-standing conventional oven (model JBP82WFWW, General Electric Co., Fairfield, CT, USA) for 20 min at 350°F. Baked cookies were cooled and placed in polyethylene bags until analysis.

Dimension and Color Evaluation

The diameter of a cookie was measured by the average of 10 evenly spaced diameters using a Central 6” Dial Caliper (Efston Science, Toronto, Ontario, Canada). The roundness of a cookie was determined by the longest diameter minus the shortest diameter, such that a perfectly round cookie will have a roundness of zero, whereas a large value for roundness indicates an out-of-round cookie. The maximum thickness was determined by taking the thickness of a cookie at the thickest point, measured relative to the belt on which the cookie sits. For thickness measurements where chocolate chips were the highest points, the data were omitted. If the cookie base was flat, this was the thickest area on the cookie. If the cookie base was curved, this was the highest point above the belt on the cookie top surface.

The bake color of a cookie (both top and bottom) was measured by the following steps. First, the cookies were placed on top of wax paper and kept at 60 cm below a 25 W GE® fluorescent lamp (model F25T8.SP41, General Electric Co., Fairfield, CT, USA) in a windowless room. Digital images of the cookies were recorded onto a SmartMedia memory card in an Olympus Camedia 1.3 megapixel digital camera (model D-450 Zoom, Olympus Optical Co., Ltd., Tokyo, Japan). The images were then imported into the Adobe Photoshop® 5.0 (Adobe Systems Inc., San Jose, CA, USA) software capable of displaying color values according to the Commission Internationale d'Eclairage (CIE) LAB color model under Windows® 2000 (Microsoft Corp., Redmond, WA, USA) for processing (Pointer and others 2002). The spectral data (L∗, ±a∗, and ±b∗, which represent lightness, redness or greenness, and yellow or blueness, respectively) and hue angle (arctan [b∗/a∗]), the angle for a point calculated from the a∗ and b∗ coordinates in the color space, were acquired from the average of 15 color readings of different spots randomly picked in the area inside a 10 mm wide guard band, an area around the perimeter of the cookie that was discarded when measuring color following the high-resolution digital imaging methods described by Kane and others.[47] Dark chip areas were avoided during measurement.

Texture and Specific Gravity Evaluation

The texture properties such as hardness of the soy cookie were determined by using a 5-kg load cell and a 36-mm diameter cylinder probe integrated with the TA-XT2i Texture Analyzer (Texture Technologies Co., Scarsdale, NY, USA). The pre-test, test, and post-test speeds were controlled at 1, 0.2, and 0.2 mm/s, respectively. The specific gravity of the cookie was measured using the Westphal balance (Arthur H. Thomas Co., Philadelphia, PA, USA), which functions on Archimedes' principle such that the plummet on the balance will be buoyed by the weight of liquid equal to the volume displaced.[48] The specific gravity of a cookie was then calculated by:

(1)

Scanning Electron Microscopy

To further elucidate and compare the structure of the cookies, several small pieces of the soy and wheat cookies were taken as samples to be examined under scanning electron microscope (SEM) at 40×. These samples were gradually dehydrated with 20–100% ethanol in increments of 10% by holding the samples at each concentration for 30 min. These samples where then cryogenically dried at the critical point with liquid CO₂. All steps, except for the critical drying, were carried out at 4°C. The completely dried samples were coated with gold/palladium before taking SEM photographs using the JEOL model 820 SEM.

Statistical Analysis

The analysis of variance (ANOVA) was performed on all values using the Statistical Analysis System (SAS) program version 6.12.[49] To compare the mean values of the results, the Student-Newman-Keuls (SNK) test was conducted at α = 0.05.

RESULTS AND DISCUSSION

Effects of Packaging Films on Cookie Appearance

The dimensional measurements of the microwave baked soy cookies enclosed in the three different types of packaging films investigated (Table 1). Soy cookies baked inside EMIR had the smallest diameter among all cookies studied, whereas the ones wrapped inside both PTFE and ePTFE turned out to be the ones with average diameters comparable to those of the wheat flour group. Only soy cookies baked with ePTFE showed excellent roundness that was considered to be the closest to that of the commercial chocolate chip cookies. Cookies baked in EMIR and PTFE had significantly greater discrepancies between the longest and the shortest diameters, leading to an out-of-round, unacceptable appearance.

Table 1 Comparison of dimension, CIELAB values, hardness, and specific gravity of microwave based soy cookies enclosed in three different types of packaging films in reference to the cookies made with wheat flour

Parameters	EMIR	PTFE	ePTFE	WFG ^†
Dimension (mm)
Diameter	51.5 ± 2.5^a	58.6 ± 3.7^b	59.4 ± 2.9^b	60.1 ± 3.2^b
Roundness	9.5 ± 1.8^a	7.4 ± 2.2^b	4.2 ± 1.6^c	4.8 ± 2.0^c
Max. thickness	10.7 ± 3.5^a	9.5 ± 3.1^b	9.2 ± 2.0^b	9.1 ± 2.2^b
CIELAB values
L∗	49.32 ± 0.43^a	51.83 ± 0.37^a	52.79 ± 0.02^a	53.24 ± 0.26^a
a∗	8.97 ± 0.04^a	4.99 ± 0.02^b	11.03 ± 0.02^c	11.57 ± 0.06^c
b∗	25.29 ± 0.22^a	23.08 ± 0.14^a	30.68 ± 0.06^b	33.58 ± 0.27^b
Hue angle (θ)	75.47 ± 0.05^a	77.80 ± 0.07^a	70.23 ± 0.03^b	70.99 ± 0.04^b
Hardness (g force)	594.5 ± 15.8^a	551.3 ± 14.2^b	572.6 ± 12.0^c	573.2 ± 13.7^c
Moisture (%)	4.97 ± 0.11^a	8.83 ± 0.06^c	7.22 ± 0.07^b	7.12 ± 0.04^b
Specific gravity	0.82 ± 0.12^a	0.42 ± 0.09^b	0.60 ± 0.11^c	0.55 ± 0.07^c
Each value is the mean ± SD, n = 10. Values bearing the same superscripts in a row are not significantly different (P > 0.05).
^† Wheat flour group (WFG) samples were prepared in conventional oven using wheat flour.

In conventional baking of cookies, physical changes in the materials were induced, including water evaporation, volume expansion, development of porous structure, and alteration in dimensions,[50] mainly caused by chemical changes such as gas formation, protein denaturation and coagulation, starch gelatinization, and crust formation and browning.[2] The conventional baking process suffers from the moisture gradient caused by migration of moisture from areas within the cookie that have high concentrations to areas that have low concentrations. The rapid drying of the surface also causes differential shrinkage near the surface, which results in mechanical stress, which, when reaches a critical point, leads to cracking.

Contrarily, microwave heating of food materials is caused by molecular friction of diploes under an oscillating electric field of specific frequency.[18] Microscopically, food matrices are inhomogeneous in structure and composition; dielectric properties thus vary spatially, which affects the conversion of microwave energy to heat.[27] Food products have macroscopically and microscopically geometric irregularity (e.g., sharp vs. round) on the outer surface; therefore, the intensity of incident microwave radiation will change at different locations. With each component in the formulation of dough specifically interacts with microwave energy,[7] if the thermal conductivity were not increased by the packaging film during microwave baking, the temperature of hot points may reach a high level and produce particular effects such as fast drying and explosion by vapor pressure, etc.,[51] thus preventing the soy cookies from reaching the desirable roundness.

Based on the measurements in the CIELAB system, the soy cookies microwaved within ePTFE showed the highest values in a∗ and b∗ (Table 1), both were comparable to those of the wheat flour group, indicating that ePTFE was capable of producing the desirable color during microwave baking of soy cookies. Soy cookies baked with EMIR, on the contrary, had the lightest color (L∗ ≈ 49.32). No significant differences in the values of L∗ were found among all the packaging films investigated. Cookies baked with EMIR and PTFE showed higher values of hue angle (θ) than those with ePTFE, indicating that the color of cookies baked with ePTFE were more reddish and less yellowish than those enclosed within EMIR and PTFE.[52] This was in agreement with the visual evidence that a nice brown color comparable to that of the wheat flour group was obtained on the cookies baked with ePTFE (Figure 1), since the values of θ were not significantly different between the cookies baked with ePTFE and the wheat flour group. Therefore, soy cookies with desirable browning could only be produced by using ePTFE as the packaging film during microwave baking.

Figure 1 Examples of the appearance and integrity of microwave baked soy cookies enclosed in different types of packaging films: (a) EMIR; (b) PTFE; and (c) ePTFE.

It is postulated that, with its hydrophobic and porous surface,[42,43] the ePTFE film was able to retain moisture around the cookies during microwave heating as the moisture content was continuously changing due to evaporation. It has been shown that the browning rate followed a zero-order reaction, with the rate constants drastically reduced with the addition of a small amount of water.[37] A generally accepted scenario of browning is that the rate of browning increases from the dry state, starting at a critical water activity of 0.2–0.3 for most foods, to a maximum at water activity of 0.5–0.8, and then decreases at higher water activities.[53] This scenario was supported by successes in applying microwave heating as a pre- or post-conventional baking process in breadmaking.[5,9,10,15] Such moisture retention capacity in conjunction with porosity might result in the development of browning evenly on the surface of the cookies. However, in depth studies are needed to understand the moisture distribution during microwave baking of soy cookies using ePTFE as the packaging film and to characterize the corresponding browning kinetics.

Effects of Packaging Films on Cookie Texture

Besides lack of browning, a poor crunchy texture in microwave based products is known to be the principal drawback for microwave baking.[16,54–56] However, in the present study, the soy cookies baked with ePTFE showed the hardness comparable to that of the wheat flour group with similar specific gravity (Table 1). Retention of excess amount of moisture was observed in cookies baked with PTFE, which has surface hydrophobicity similar to that of ePTFE but with much less porosity.[44] The cookies baked with EMIR had highest hardness, much harder than that of the wheat four group, and a very high specific gravity when compared with the wheat flour group. EMIR also had the poorest moisture retention among all packaging films studied. Conversely, cookies baked with PTFE were the softest, as indicated by the lowest value of hardness and specific gravity. The pores on the surface of cookies baked with EMIR and PTFE were much more obvious than that with ePTFE (Figure 1).

Moreover, the cookies baked with PTFE formed a vacuole at the center of the cookie, which in turn resulted in a reduced specific gravity (Table 1). When observed under SEM (Figure 2), only the cookies baked with ePTFE showed continuous, integral structure. While many web-like holes were seen in the cookies baked with EMIR, a fluffy, hollow structure was observed in those baked with PTFE. Based on these results, a soy cookie product with appearance and texture comparable to those of regular chocolate chip cookies could only be produced by using ePTFE as the susceptor to enclose soy dough during microwave baking. The ePTFE surface hydrophobicity and porosity resulted from further stretch of PTFE[42,43] are most likely responsible for the enhanced appearance and texture of the cookies during the microwave baking process.

Figure 2 SEM images(40×) showing the top surface of (a) a whole wheat cookie baked in a conventional oven and microwave-baked soy cookies enclosed in different types of packaging films: (b) EMIR, (c) PTFE, and (d) ePTFE.

It is important to note that the soy flour used in this study was lecithinated (with 15% addition) to prevent the cookies from falling apart during processing,[57,58] since it has been shown that the presence of emulsifier, in addition to fat, is crucial to reduce the weight loss during microwave baking.[7] This is because lecithin is capable of delaying the gelatinization of starch, which is known to affect the volume of baked products.[59] Without the addition of lecithin as an emulsifier, the soy cookies baked in microwave oven had totally unacceptable textures. The exterior parts were tough and flaky and the interior parts were firm, elastic, and difficult to chew (data not shown). Another important aspect with microwave heating is that, when heating dough in a microwave, most of the microwave energy is absorbed during the first few seconds by the susceptor due to its high dielectric loss factor and small mass.[45,46] The product starts to heat up after this time and absorbs part of the energy, reducing the amount of energy absorbed by the susceptor. During longer heating periods (about 50 sec), the energy absorbed by the susceptor again increases and then declines after about 120 sec due possibly to susceptor damage.

Resulted from the absorption of microwave energy by water molecules, which are the most abundant dipole component of foods, and other components such as salt, fat, and protein that also act as dielectric components,[35] microwave heating of a food product in a microwave oven is expected to be more even with than without proper retention of moisture during baking. Hence, for cookies baked with PTFE, the moisture evaporated from the dough was reabsorbed by the cookie during the cooling period, leading to a texture softer than that of the wheat flour group. The formation of a hollow center in cookies baked with PTFE could be attributed to uneven moisture distribution in the dough, since the moisture was reabsorbed from the surface of the cookies and could not reach the center of the dough due to the strong evaporation that continued to force the moisture out.

CONCLUSIONS

Among all three types of packaging films investigated, ePTFE was capable of producing soy cookies with the appearance and texture attributes comparable to those of commercial chocolate chip cookies made of wheat flour. Use of lecithinated soy flour, salts, and sugar was essential in the formulation of the soy cookies, as the presence of lecithin as an emulsifier is critical in reducing weight loss during microwave baking. The surface hydrophobicity and porosity of ePTFE were suggested crucial in reaching the degree of browning, the hardness, and the specific gravity of the product. In depth studies are recommended to understand the moisture distribution and migration during microwave baking of soy cookies using ePTFE as the susceptor, while the corresponding kinetics for surface browning and crust formation remain to be investigated.

ACKNOWLEDGMENTS

Research support was provided in part by the Soybean Boards of Delaware and Maryland. Appreciation is extended to Dr. Robert Keown for insightful discussions during experimental design and planning.